Control of the Metabolic Flux through the MEP Pathway

Plants produce an enormous variety of metabolites which can be categorised into those responsible for primary metabolism and those synthesizing secondary metabolites. Although secondary metabolites are not directly involved in the plant’s growth and development, they are essential for the plants survival through ecological interactions with the environment. A very large amount of secondary compounds are synthesized by the modification of common backbone structures. This structural variation of secondary metabolites is important for the ability of plants to adapt to environmental pressures such as pests and diseases. The isoprenoids (also called terpenoids) are the most diverse group of secondary metabolites with tens of thousands of compounds identified. Certain isoprenoid compounds are also involved in primary metabolism with functions in respiration, photosynthesis, and regulation of growth and development. Many isoprenoids are also involved in the defence response of plants, including indirect defence through the emission of volatile signals that can attract herbivore predators. Despite their diversities, all isoprenoids are biosynthesized from the same five-carbon (C5) isoprene building blocks, isopentenyl diphosphate (IPP) and its isomer dimethylallyl diphosphate (DMAPP). It is to be expected that a metabolic pathway responsible for the biosynthesis of such a wide variety of compounds should be finely regulated to meet the demands of different tissues, organs and growth stages.

Catabolism of exported MEP intermediates

It was shown recently that 2-C-methylerythritol-2,4-cyclodiphosphate (MEcPP) acts as a retrograde signal in plastid-to-nucleus communication. This MEcPP mediated signaling activity is transient and occur under specific abiotic stresses. It stands to reason that MEcPP should be converted to another form to fulfil its signaling function. We are therefore investigating the catabolism of MEcPP after its export out of the MEP pathway.

Regulation of the MEP pathway in isoprene emitting plants

It is known that some plants emit isoprene in big amounts, while isoprene emission is totally absent in others. Isoprene is biosynthesized through the methylerythritol phosphate (MEP) pathway, which is also responsible for the synthesis of many important molecules involved in primary and secondary metabolism in plants. The MEP pathway is also a component of the defence signal cascade in plants and responsible for the emission of other ecologically important terpenoid volatiles. It is therefore important to understand the regulation of the MEP pathway and its influence on the emission of volatiles. Since isoprene emission is responsible for the majority of the flux through the MEP pathway in isoprene emitting plants, it stands to reason that the regulation of the MEP pathway in these plants will differ from non-emitting plants. This project aims to elucidate the regulatory steps involved in isoprene biosynthesis and the interaction between the MEP pathway, isoprene emmission and the environment. Furthermore, the regulation of the MEP pathway in Populus species, as isoprene emitter, will be compared with the regulation of the MEP pathway in the isoprene non-emitting plant Arabidopsis thaliana.

Regulation of the MEP pathway in isoprene producing conifers

The plastidial methylerythritol phosphate (MEP) pathway synthesizes isoprenoids important for plant metabolism, photosynthesis, and biotic and abiotic defenses. They are synthesized from the five-carbon (C5) isoprene building blocks, isopentenyl diphosphate (IPP) and its isomer dimethylallyl diphosphate (DMAPP). Isoprene, the most common biogenic volatile on earth, is the simplest product of the pathway, and it is also believed to play an important role against abiotic stress like drought. Rising global temperature coupled with reduced precipitation will lead to longer and more frequent dry periods (IPCC 2013), resulting in more frequent and intense droughts. We are thus interested to understand the effect of drought on isoprene biosynthesis and how the MEP pathway is regulated under these conditions. Although conifers are dominant species in the immense boreal forests in the northern hemisphere, producing huge quantities of isoprene, very little is known on its regulation. To fill this gap on our knowledge of global isoprene production, this research will focus on the isoprene producing conifer, Picea glauca. With this work we also hope to understand the purpose of isoprene emission during drought stress and in particular its influence on the metabolism of MEP pathway derived isoprenoids.